Single crystal wafers

ABSTRACT

Improved commercial single crystal wafers (250), as shipped to end users form a full circle, and comprise a &#34;stress concentration notch&#34; (172) which accurately defines a desired cleavage plane. The stress concentration notch is introduced into the wafers in bulk by means of a properly oriented cut along the length of a single crystal ingot, after machining the ingot to the desired end product diameter, and prior to sawing the ingot into slices. The stress concentration notch uniquely defines the first and second faces of the wafer.

TECHNICAL FIELD

This invention relates to improved wafers prepared from single crystalingots and to a method of manufacture thereof

BACKGROUND OF THE INVENTION

Wafers which are prepared from single crystal ingots of a variety ofmaterials are used in manufacturing applications which require divisionof a wafer into components by cleaving along natural cleavage planes ofthe wafer. GaAs, GaN, AlN, and InP are representative examples of usefulwafer materials for such applications; and the manufacture of lasers isa relevant example of a manufacturing application.

In the prior art, wafers, which are provided to a manufacturer, have amajor Orientation Flat, termed an "OF" herein, that defines a selectedset of natural cleavage planes of the wafer. In a first form, the wafershave orthogonally oriented major and minor Orientation Flats that aregenerated by grinding the ingot before slicing. A second form of wafercomprises a cleaved major OF and a ground minor OF.

The purpose of the minor flat is to, in combination with the major flat,uniquely identify the two plane surfaces of the wafer. For example, if,in geographic terms, the major flat is placed to face south, and theminor flat faces west, the surface of the wafer facing the viewer is thefirst surface. Similarly, if the major flat is placed to face south andthe minor flat faces east, the surface facing the viewer is the secondsurface. The ability to uniquely determine the two plane surfaces of awafer is useful in all cases, and is extremely important in the cases ofwafers in which both faces are polished.

Unfortunately, ground Orientation Flats are lacking in precision intheir definition of natural cleavage planes. When a wafer with groundflats is to be used in an application that requires high precision inthe definition of cleavage planes, the wafer must be scored and cleavedparallel to the ground OF at a distance sufficient to permit the waferto be held during cleaving, e.g., 5 to 6 mm between the ground OF andthe first cleaved plane. Thus, the portion of the wafer between theground flat and the first line of cleavage is lost.

Because of the high cost of wafers and of component processing, any lossof wafer surface is a serious economic problem.

Furthermore, post processing of finished wafers to produce components iscomplicated by the presence of a reference flat. For example, where postprocessing includes deposition of an EPI layer, the flat causes anunwanted pattern of gas flow around the wafer, differential dissipationof heat from the wafer, and resulting mechanical deformation of thewafer. Additionally, unless the reference flat is protected duringgrowth of an EPI layer, the layer tends to crown adjacent the flat andthus reduces the accuracy of the flat as a reference.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, our improved commercialwafers, as shipped to an end user, form a full circular plane andcomprise a relatively narrow, shallow, and soft bottomed "stressconcentration notch" which facilitates cleavage at a desired point inthe post processing of such wafers to produce components. While thenotch facilitates cleavage, the mechanical strength of the wafer isretained and there is little or no chance of premature accidentalcleavage prior to or during post processing.

Advantageously, our present invention can be employed with waferscomposed of any single crystal material that exhibits families ofnatural clevage planes.

Advantageously, our improved wafers can be processed to form componentswithout the surface losses incurred with wafers which comprise groundOrientation Flats, and without the described complications incurred ingrowth of an EPI layer on wafers with Orientation Flats. Additionally,since our stress concentration notch is not pointed toward the center ofthe wafer surface, the position of the notch fully defines which surfaceof the wafer is facing a viewer. For example, if the wafer is orientedso that the notch runs from west to east and is located in the southwestquadrant of the wafer, the viewer is facing the "first" surface.Similarly, if the wafer is oriented so that the notch runs from east towest and is located in the southeast quadrant of the wafer, the vieweris facing the "second" surface. The ability to locate the first andsecond plane surfaces is important in most cases, and vital in the caseof wafers with two polished faces.

Wafers in accordance with our present invention are produced as follows:

(1) Ingot is machined to a perfect cylinder at near the desired endproduct diameter;

(2) A desired natural cleavage plane of the ingot is determined;

(3) A shallow, soft bottom, narrow cut, aligned with the determinedplane, is made along the length of the ingot;

(4) Ingot is sliced into wafers:

(5) Edges of the wafers, as required, are ground to the finisheddiameter;

(6) Edges of the wafer are bevelled;

(7) Wafers are polished;

(8) Wafers are: cleaned;

(9) Wafers are packaged & shipped to an end user.

Production of wafers in accordance with the present invention,economically provides improved commercial wafers without loss ofaccuracy in the definition of the reference cleavage plane.

THE DRAWING

FIG. 1 is a perspective view of an ingot with a longitudinal cut inaccordance with the present invention;

FIG. 2 is a top view of an improved wafer sliced from the ingot of FIG.1; and

FIG. 3 is an exploded view of the stress concentration notch 172.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of an ingot 150 used in the preparation ofthe wafers of FIG. 2. The orthogonal lines 151 represent any selectedset of natural cleavage planes of ingot 150 and of the resulting wafers250 sliced therefrom. Although the diameter and length of an ingot arenot critical to the practice of our present invention, an ingot, by wayof example, may have a two inch diameter and a length of four to sixinches. The figures of the drawing are not made to scale.

Wafers in accordance with the present invention are produced as follows:

(1) Ingot 150 is machined to a perfect cylinder at near the desired endproduct diameter;

(2) A selected reference cleavage plane 161 of ingot 150 is determined;

(3) A cut 171, aligned with the selected cleavage plane 161, is madealong the length of the ingot;

(4) Ingot is sliced into wafers 250;

(5) Edges of the wafers, as required, are ground to the finisheddiameter;

(6) Edges of the wafer are bevelled;

(7) Wafers 250 are polished;

(8) Wafers are cleaned;

(9) Wafers are packaged.

The manufacture of wafers in accordance with our present invention ismore economical than prior art processes; and results in an enhanced enduser products which admit of simplifications in post processing toconstruct components from such wafers.

For example, where post processing of a wafer by a manufacturer includesdeposition of an EPI layer, the use of full circle wafers avoidsunwanted patterns of gas flow around the wafer; differential dissipationof heat from the wafer; and the resulting unwanted mechanical distortionof the wafer. Furthermore the manufacturing process is simplified sincethere is no flat to protect.

FIG. 2 is a top view of a commercial wafer 250, in accordance with thepresent invention. Our improved wafer comprises a full circular planewith a "stress concentration notch" 172 formed by longitudinal cut 171.By way of example, cut 171 may be formed by a saw cut, a wire cut, ormay be formed by laser action. Solely, by way of example, notch 172 maybe a few mils wide, and 2 to 3 millimeters deep. Advantageously, notch172 includes a radius 271. Radius 271 reduces any tendency of the waferto prematurely cleave due to the presence of notch 172. During postprocessing, a small score is made in the wafer near the end of notch 172at the point in time that the initial cleave is to be made. Thereafter,when the wafer is flexed parallel to the desired cleavage plane, notch172 facilitates cleavage by concentrating the forces applied at or nearthe score.

The invention has been described with particular attention to itspreferred embodiment; however, it should be understood that variationsand modifications within the spirit and scope of the invention may occurto those skilled in the art to which the invention pertains.

What is claimed is:
 1. A single crystal wafer (250) comprising:a circular plane body comprising: a finished edge; first and second major plane faces, and a stress concentration notch (172), said stress concentration notch (172) being located at said finished edge and extending through said major faces along the line of a selected natural cleavage plane of the wafer.
 2. A single crystal wafer in accordance with claim 1characterized in thatsaid stress concentration notch (172) forms a radius (271) at the bottom thereof.
 3. A single crystal wafer in accordance with claim 1characterized in thatsaid finished edge is bevelled.
 4. A single crystal wafer in accordance with claim 1characterized in thatsaid wafer is composed of Gallium Arsenide.
 5. A single crystal wafer in accordance with claim 1characterized in thatsaid wafer is composed of Indium Phosphide.
 6. A single crystal wafer in accordance with claim 1characterized in thatsaid wafer is composed of any single crystal material that exhibits families of natural cleavage planes. 